Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

Muscles always work in conjunction with one another. In this lesson, we'll explore the definition of an antagonist muscle, along with some examples of how antagonist muscles work.

Muscles in Conjunction

For every action, there must be an equal and opposite reaction. The same is true for muscles! Think about moving your arm, for example. If you bend your arm at the elbow, one muscle is tightening to pull your arm up; another muscle is working in tandem, relaxing to counterbalance the first muscle. When you relax your arm, the muscles take on opposite roles to pull your arm straight.

We call these opposites agonist muscles, or muscles that produce movement through contraction; and antagonist muscles, which are muscles that provide the opposite of the agonist movement. Sometimes, antagonist muscles control and slow down movement opposite to their agonist partner, while in other situations a muscle can be an antagonist throughout a particular movement. The terms agonist and antagonist aren't set properties of a muscle; they apply to a muscle depending on whether the muscle is doing the movement.

We'll use the terms extension and flexion when discussing antagonist muscles and their actions. Extension is the action of increasing the angle of corresponding bones, such as straightening your arm; and flexion is the action of decreasing the angle of corresponding bones, like bending your arm at the elbow.

Definition of Antagonist Muscle

In your upper arm, there are two main muscles. Go ahead and make an arm curl to see how big your biceps, or upper arm muscle, is.

While you are squeezing and contracting your biceps muscle to flex your arm, the biceps is carrying out the main movement, and so it is the agonist muscle. There's another muscle on the underside of your upper arm, called the triceps, or lower arm muscle. The triceps in this case is the antagonist muscle, relaxing and providing movement control while the biceps does the main contraction and movement. An antagonist muscle, just like the antagonist in a novel, works opposite to the main character, which in this case is the agonist muscle that undertakes the main action of movement.

More Examples of Antagonist Muscles

There are several other examples of antagonist muscles in the body. The first example is pretty simple: what happens if we extend our arm and relax it so it's laid out straight? Can you guess which muscle, triceps or biceps, is the antagonist?

If you guessed the biceps, you're right! But how did the biceps become the antagonist, when it was just the agonist in the first example? Well, when we have two muscles performing opposite actions, the muscles change from agonist to antagonist depending upon which muscle is performing the action. Which muscle was performing the pulling action to extend your arm? The triceps! That makes the triceps the agonist when you extend your arm out straight. And, simultaneously, the biceps becomes the antagonist muscle for this particular action.

What other examples of antagonist muscles exist throughout the body? Think about other parts of the body where a series of bones can be flexed or extended. There are antagonist muscles in our legs, such as the gastrocnemius muscle, a big muscle that sits in the calf of our leg. The gastrocnemius is an agonist when it pulls to bend our leg at the knee, but it is the antagonist when the leg is straightened. The muscle that works opposite to the gastrocnemius is called the tibialis anterior, and it runs along our lower leg at the shin.

Another leg example of an antagonist muscle and its paired agonist is in the upper leg. The muscles at the front of our upper legs are called the quadriceps muscles, and they act as an antagonist when you lift your leg up high. This is also known as flexing the hip. When you relax your leg down towards the ground again, or extend the hip, it's the quadriceps muscles that become the agonists. The muscle and tendon combination that performs the opposite action to the quadriceps is known collectively as the hamstrings. A tendon is the elastic-like connective tissue that connects muscles to bones. The quadriceps and hamstrings also play a role in bending the knee and rotating the leg.

All of these muscles form a larger and more complex interaction; in fact, while the gastrocnemius is flexing the knee, the quadriceps muscles also help to flex the knee. Muscles don't always work in easy, opposite pairs, but we can categorize their actions based on which muscle is contracting in relation to another muscle.

Try performing these actions and think to yourself: Which muscle is the agonist, and which is the antagonist? Remember, it's all relative to the action you're performing, and the roles switch as soon as you swap movement to the opposite direction. Your own body is your study guide!

Lesson Summary

Muscles always move in pairs, and the way in which they move can be described based on which muscle is doing the contracting, or pulling. An agonist muscle always performs the contracting movement, while the antagonist muscle serves as the counter muscle, relaxing and often balancing the movement of the agonist muscle.

The antagonist and agonist actions of muscles occur when muscles interact to move bones. Extension describes muscle action that increases the angle of corresponding bones, such as straightening your arm out flat. Flexion is used to describe muscle action that decreases the angle of corresponding bones, such as bending your arm at the elbow.

There are several examples of agonist muscles, as well as their corresponding antagonists, within the human body. One main example is the movement of the biceps in the upper arm and the triceps in the lower arm, which counters the biceps movement. Other examples include the opposite movements of the gastrocnemius, a big muscle that sits in the calf of our leg, and the tibialis anterior, the muscle that works opposite to the gastrocnemius in the lower leg, which helps to bend and straighten our leg at the knee. In the upper leg, the quadriceps, the muscles at the front of our upper legs, and the hamstrings, the muscle and tendon combination that performs the opposite action to the quadriceps, flex and extend the leg at the hip and also help to flex and extend the knee. Many of these agonist-antagonist muscle movements are also dictated by the tendons, the elastic-like connective tissue that connects muscles to bones.

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